Preparation of polymeric membranes for fluid and gas separation applications are well documented in the art. In order for a polymer to qualify as a suitable membrane forming and fluid separation material, it must meet several performance criteria that depend on the desired end use of the membrane. Among the factors that will influence the choice of a polymer are its mechanical strength, chemical resistance, thermal stability, and most importantly its separation and permeation characteristics. In addition to the aforementioned considerations, it is frequently preferred that a prospective membrane polymer be commercially available at a moderate cost.
There are numerous references in the literature to membranes manufactured from polyimides. Makino et al. teach preparation of several specialty polyimides and membranes produced therefrom in U.S. Pat. Nos. 4,440,643; 4,460,526; 4,512,893; and 4,528,004. Chung et al. describe preparation of asymmetric hollow fibers for gas separation from fluorine-containing polyimides in the Journal of Membrane Science, 75 (1992), 181-195. This work is an example of a gas separation membrane fabricated from an expensive, custom synthesized polymer.
The fabrication of gas separation membranes from various polyimides has also been taught by Hoehn at al. in U.S. Pat. No. 3,899,309 and by Hayes in U.S. Pat. Nos. 4,838,900; 4,880,442; 4,932,982; and 4,932,983.
Two commercially available polyimides that are of interest as membrane forming materials because of their superior strength and chemical resistance are P84 and P84HT, manufactured by Lenzing A. G. The former material is reportedly a product of the reaction of benzophenone tetracarboxylic dianhydride (BTDA) with toluene diisocyanate (TDI) and methylene di p-phenyl diisocyanate (MDI). The latter polymer is reportedly a product of the reaction of pyromellitic dianhydride (PMDA) and BTDA with TDI and MDI. The polymers will hereinafter be referred to as BTDA/TDI-MDI and BDTA-PMDA/TDI-MDI polyimides respectively.
Another material with more advantageous separation properties is a polyimide that incorporates phenylindane moieties in the polymer chains. An example of such polyimide is Matrimid 5218, commercially available from Ciba Geigy Co. The polymer exhibits good combinations of gas permeability coefficients and separation factors for many gas pairs. The fabrication of asymmetric membranes from this polyimide has been reported by Wang et al. in U.S. Pat. No. 5,067,970. Ekiner et al. disclose the use of phenylindane-containing polyimides to prepare gas separation membranes in U.S. Pat. No. 5,015,270. While membranes described in the aforementioned patents display good gas separation characteristics, commercial use of these membranes can be limited because of the high cost of this specialty polymer.
The practice of blending polymers has been used effectively in the formation of gas separation membranes. Burgoyne, Jr. et al. in U.S. Pat. No. 5,061,298 disclose the use of blends of polyimide polymers as part of a process to prepare air separation membranes as shown. Yamada et al. in U.S. Pat. No. 4,832,713 disclose fabrication of gas separation membranes from blends of polyetherimide mixed with materials such as polycarbonates or polysulfones. Ekiner and Simmons, in U.S. Pat. No. 5,248,319 disclose the preparation of gas separation membranes from blends of polyimides in which one of the polymers contains phenylindane residues. The other polymer in the blend must incorporate aromatic diamine moieties that contain ether linkages. The selection of polyimide polymers that can be blended with phenylindane-containing polyimides according to the teachings of this patent are therefore severely limited.
It was discovered by us that integral anisotropic fluid separation membranes with superior combination of separation/permeation characteristics can be prepared from blends of BDTA/TDI-MDI and/or BTDA-PMDA/TDI-MDI polyimides and phenylindane-containing polyimides. These membranes can be utilized in preparation of porous substrates useful in the manufacture of composite membranes by solution coating processes.